Re: about ldo?????????????
Short current loop is the case when you connect VCC wire to GND and
resistance between VCC and GND = 0.
( it's happen on PCB when you accidentally short trace by probe or screwdriver, or when there is solder bridge between traces , or when your schematic is wrong and there is two bjt, one connected to VCC, other to GND and both connected with each other and open in the same time )
In this case LDO connected to VCC see then (Ohm law) request for current is :
I = VCC/R = VCC/0 = infinite
And try to provide infinite current. Next, maximum LDO current, flowing through
our short wire, warming that wire and eventually burn LDO or wire ( win strongest).
Actually take place next processes :
LDO works as constant voltage source (not current), when you connect to LDO
wire with very small resistance , say 0.1ohm , LDO try to send very small current , say 0.1 ma, and LDO see on his output voltage :
V = I*R = 0.1ma*0.1Ohm = 0.0001V
But LDO know, then he must provide 3.3V and try to perform this task and
adds some current to output , say 10 ma :
V = I*R = 10ma*0.1Ohm = 0.001V
This is too small , LDO adds more , 100ma :
V = I*R = 100ma*0.1Ohm = 0.01V
Let LDO maximum current 1.25A , he give all him current :
V = I*R = 1.250ma*0.1Ohm = 1.25*0.1 = 0.125V
And LDO remain in this state, attempting to provide 3.3V to him output.
Well now calculate Power dissipation :
Power on short resistor (0.1ohm) = 0.1*1.25^2 = 0.15w this is not much
Power on LDO (input 5V) = (5V-0.125V)*1.25 = 6w this is toooo much
30 seconds and your LDO is dead, if he has not current short protection.
There is LDO with current short protection .
For your schematics , i think, very suitable LDO from Linear Technology
www.linear.com LT1763 :
Output Current: 500mA
Wide Input Voltage Range: 1.8V to 20V
No Protection Diodes Needed (if you change input polarity)
Fixed Output Voltages: 1.5V, 1.8V, 2.5V, 3V, 3.3V, 5V
Adjustable Output from 1.22V to 20V
Reverse Battery Protection (if you change output polarity)
Overcurrent and Overtemperature Protected ( this is case current short loop)
you can request free sample from the site, it's work.